Composition and method for treating aluminum and aluminum alloys



Oct. 4, 1955 F. H. HESCH 2,719,781

COMPOSITION AND METHOD FOR TREATING 1 ALUMINUM AND ALUMINUM ALLOYS Filed April 9, 1952 N fsTISFAQTQ Y s33. G IN 20 SJQ. Loss 5ND P, -25- 9: END I M 16 1 Ti 355 pf 16 g5 14 5. .GAIN I g H no TQHING I q \SOME i-lfSZIjNjEZss I m 12 z I1 I o 10 5.361517! 0 Nomcams I Z N flfillflil 8 m Q: A) I E O I T ll l l l l I l o\ 10 so so G1 3 06- F 70 "7 W103 PERLITR ILECIEND G 3IJ IN SPECULBR REFLECTHNCE PBRfiLl-EL TO ROLLING DIRECT 101*! SOLUTION COMPOSITION INVEN TOR. FRE ER CZB amen fifi' fifi BTTORNEZY United States Patent COMPOSITION AND METHOD FOR TREATING ALUMINUM AND ALUMINUM ALLOYS Frederick Harold Hesch, Anaheim, Calif., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Application April 9, 1952, Serial No. 281,352

19 Claims. (CI. 41-42) This invention relates to the brightening of aluminum and aluminum alloys. More particularly, the invention relates to a composition and method for chemically brightening aluminum and aluminum alloy surfaces.

Although aluminum is ordinarily considered a bright metal, it often presents a dull or matte-like finish due to the oxide films inevitably formed on its surfaces during processing. In addition, wrought surfaces exhibit fabrication marks, e. g. roll marks usually extending parallel to the rolling direction. Such fabrication marks greatly influence the light reflecting characteristics of the surfaces. Many aluminum and aluminum alloy products are fabricated from mill finished sheet or other wrought materials having such a characteristic dull and marked appearance, which leads to a demand by fabricators for means to impart to the metal surfaces a bright, lustrous finish having a relatively high specular reflectance.

Specular reflectance of a bare metal surface is a measure of the smoothnes or rather flatness of the surface, whereas total reflectance is a property of the material or composition of the surface, and is of little value in indicating flatness. However, specular reflectance may perhaps be more accurately defined by reference to the measurement of the quantity. A light beam is projected on the surface at a given angle (e. g. 60) to the apparent plane of the surface. Part or all of the incident light is reflected by the surface to a photocell located so as to receive all of the light reflected by elements of the surface that make the given angle (e. g. 60) with the direction of the incident beam. Irregularities in the surface do not reflect light in the direction of the photocell, and thus the more irregular the surface the lower is the portion of total incident light received by the photocell. The photocell is connected to a voltmeter which directly measures the percentage of incident light reflected. Thus, the specular reflectance of the surface may be measured by comparison with a standard assumed to be a perfect reflector, e. g. a plate glass mirror, by adjusting the meter reading to 100% using the standard. For measurement of increments or gains in specular reflectance (hereinafter at times refererd to as gains in S. R. for purposes of brevity), no assumption is involved and the plate glass mirror is merely a convenient base or standard of specular reflectance.

Successful brightening of aluminum and its alloys involves the necessity of obtaining relatively large or significant, and preferably maximum, gains in specular reflectance on the treated material. However, in chemical brightening, it has been determined that gains in specular reflectance often cannot be produced without the metal exhibiting a visibly etched or pitted condition or presenting a hazines which markedly detracts from the appearance desired. Brightened stock of this character is generally unsuitable for the requirements of a visibly smooth, bright surface of high specular reflectance.

One widely used method of polishing aluminum and its alloys is by mechanical bufling with a suitable abrasive. However, this mechanical polishing although productive 2,719,781 Patented Oct. 4, 1955 of good results in terms of bright finish of high specularity is expensive and is not easily adaptable to articles having intricate shapes and inaccessible surfaces. Other known methods such as electropolishing and electrobrighteniug involve subjecting the aluminum article to an electrolytic treatment in an appropriate solution and produce a very desirable finish of high specular reflectance. However, the methods are both slow and prohibitive in cost for many end uses of the metal.

Chemical brightening of aluminum and its alloy has previously been accomplished in some measure using a solution of nitric and hydrofluoric acids. However, consistently reproducible results in terms of suitable gains in specular reflectance of the surface while avoiding either gross or fine etching, pitting, or haziness have not been obtainable with this two component solution following the teachings and practices of the prior art. As one indicative instance, it has been determined that a given high purity aluminum, specifically, 99.8%, exhibits a large loss in specular reflectance when treated in a nitric acid-simple fluoride solution even though previously determined optimum concentrations of fluoride and nitric acid were employed. Furthermore, on aluminum and aluminum alloys in general, it was found according to the invention that acceptable gains in specular reflectance with simultaneous avoidance of etching, pitting, haziness, or deposition of an objectionable black coating on the metal could be consistently obtained only by a simple fluoride-nitric acid solution wherein the absolute concentrations and relative proportions of these two components are maintained within certain critical limits, and the two component solution is modified by the presence therein of a third constituent, whether derived in the first instance from the metal being treated or by positive addition to the solution.

Accordingly, it is a primary object of the invention to provide an improved simple fluoride-nitric acid brightening composition whereby chemical brightening to produce a desirable specular reflectance may be consistently accomplished on aluminum and aluminum alloys, without any marring of the appearance of the surface by pitting, etching or haziness.

It should be noted, as stated above, that measured gains in specular reflectance from that on mill finish stock can be obtained with a simple fluoride-nitric acid dip even though the surface of the metal presents a visibly etched, slightly pitted or hazy appearance. This mars the surface and detracts from the general appearance, thus rendering the treated metal more or less unacceptable for many decorative applications. Therefore, an other object of the invention is to eliminate this disadvantageous combination of results often obtained with the prior art hydrofluoric acid-nitric acid brightening solutions by provision of an improved composition and process for chemical brightening of aluminum and aluminum alloys.

A further object of the invention is to provide a chemical brightening composition of the simple fluoride-nitric acid type which is characterized by its economy in use, being a relatively dilute solution with low drag-out losses, and capable of producing surfaces of high specular reflectance combined with a clear, visibly smooth appearance.

The invention also provides a process for chemical brightening of aluminum and its alloys to produce surfaces of high specular reflectance and devoid of etching, pitting, haziness or the deposition of an objectionable black coating having adverse effects on the treated metal appearance even though said coating is subsequently removable.

Other objects and advantages of the present invention will become apparent from the detailed description set forth below.

It was discovered that the production of acceptable gains in specular reflectance without etching, pitting or haziness are consistently obtained with a composition comprising an aqueous acid solution containing fluoride ions and nitric acid in certain well-defined proportions and ranges of concentrations, and a small but effective concentration or amount of a dissolved metal electropositive to hydrogen.

The term electropositive as used in the specification and appended claims is based upon the definition and values of single electrode potentials on the hydrogen scale given in the Chemical Engineers Handbook, John H. Perry, Editor-in-Chief, 2nd edition, eighth impression (1941) (McGraw-Hill Book Company), pages 2746 to 2748, inclusive. It is there indicated that In solutions containing their own ions, noble metals (e. g. with electrolytic solution pressures lower than that of hydrogen) acquire a positive potential, while base metals (e. g. with electrolytic solution pressures greater than that of hydrogen) acquire a negative potential.

The presence of effective amounts of dissolved metal electropositive to aluminum as a constituent of the aqueous brightening solution may readily be accomplished by the addition of a soluble salt of the selected metal or the dissolved metal may be introduced into the solution from the aluminum or aluminum alloy upon exposure thereof to the fluoride-nitric acid solution, when the aluminum or alloy contains the electropositive metal in sufficient quantity.

Copper is the preferred electropositive metal from the standpoint of cost while producing optimum results together with the fact that when the metal is supplied from the aluminum or aluminum alloy, it is more or less invariably copper, present as a positively added alloying constituent or as a residual.

It has been determined that in general the desired results are obtained with a concentration of dissolved copper, i. e., copper ions, not substantially less than about 0.001 grams per liter (l.6 l molar). Other suitable electropositive metals may be employed when present in minimum concentrations approximately equivalent to that above-set forth for copper. For example, silver and other metals electropositive to hydrogen whose nitrate salts are sufliciently soluble (i. e. adequate solubilities in nitric acid) are productive of satisfactory results.

Although it has been indicated that the aluminum or aluminum alloy alone may suflice for the required concentration of the electropositive metal, the invention includes the positive addition of such metal to the solution in soluble form regardless of the composition of the aluminum being treated. On the other hand, consistently good results are obtained without positive additions of copper or other electropositive metal. when copper is present in the aluminum or aluminum alloy in amount at least corresponding to the lower portion of the range normally present as a residual in commercially pure aluminum, for example, 0.05%. In tests recorded below, excellent results were produced with 2SH12 aluminum alloy containing only 0.1% copper by weight. However. improvement in the specular reflectance gains are observed upon increasing the copper ion concentration up to about 0.1 gram per liter (0.4 gram per liter (h1(NO3)2-3H2O).

The copper content of the solution however, is not limited to the above-mentioned concentration, and much higher amounts may be used without any adverse effects. For example. about 2 grams per liter (about 8 grams per liter of Cu(NO3) -3H2O as a positive addition), which is twenty times the above-mentioned figure, has been used with excellent results. Also the aluminum alloy may contain copper in amount producing much higher dissolved copper concentrations without adverse effect. In regard to positive additions, however, it serves no useful purpose, and is uneconomical to increase the dissolved coplit per concentration above about 0.1 gram per liter. The foregoing statements apply to equivalent amounts of other positively added electropositive metals.

The suitable form for positive addition of the electropositive metal is any soluble salt compatible with the solution, for example, one wherein the anion will not form a precipitate with other bath constituents, and will not adversely affect brightening. In order to simplify the solution composition, it is preferred to add the copper or other electropositive metal as a nitrate. It may, however, be added as a fluoride, provided that the fluoride ion concentration and fluoride-nitric acid proportions are maintained within the critical limits defined below.

The invention further provides for certain critical concentrations of nitric acid, both absolute and relative to fluoride which must be observed to obtain the desired results set forth above. The nitric acid is the primary source of the hydrogen ions necessary for the oxidation (dissolution) of the aluminum which must occur to produce leveling of the surface and the consequent gains in specular reflectance. This reaction may be illustrated as follows:

It has been found that the nitric acid content of the brightening solution should be from at least about 3 cc. per liter up to about 106 cc. per liter HNO3 (70% by weight). Within this range, when properly correlated with the concentration of fluoride, as set forth below, substantial gains in specular reflectance are obtained without etching or pitting. The preferred range for nitric acid concentrations is from about 3 to about 55 cc. per liter HNO: by weight). Within this range, optimum specular reflectance is obtained when the proper ratios of HNOs to fluoride are observed and haziness as well as etching and pitting is avoided.

The fluoride component of the brightening solution may be any soluble simple fluoride which does not produce an objectionable precipitate with the aluminum ion introduced into the solution by metal dissolution, as indicated by the foregoing equation. It is preferred to employ ammonium fluoride, although ammonium bifluoride, hydrofluoric acid in equivalent amounts in respect to fluoride are productive of equally good results. In the case of hydrofluoric acid, the slight additional H ion concentration is of no consequence. The ammonium fluoride is preferred because of ease of handling, and cost. Sodium fluoride and other alkali metal fluorides are usable but their aluminum complex salts are not as soluble, and as the aluminum ion concentration increases there may be a tendency toward formation of a precipitate.

The fluoride concentration may best be expressed as fluoride ion stoichiometrically equivalent to a stated amount of ammonium fluoride as the soluble salt added in preparing the solution.

The effective concentrations of fluoride ion in terms of ammonium fluoride are from about 0.5 gram per liter to about 25 grams per liter in the proper ratios with the amount of nitric acid employed. The preferred concentrations for highest specular reflectance without haziness as well as avoidance of pitting or etching are from about 0.5 to about 13 grams per liter.

As indicated above, it was found that not only must the above concentrations of nitric acid and simple fluoride be observed, but the proper relative proportions of HNOa to fluoride must also be observed to obtain high specularity with no etching, pitting or haziness. In addition, it was determined that in certain solutions to which no positive copper was added, a black coating was deposited on the aluminum surface. Although readily removed in a subsequent nitric acid dip, the presence of the black coating during brightening prevented attainment of specular reflectance gains of the order possible with solutions in which the black coating formation was avoided. In addition, test panels on which the coating appeared exhibited more haziness. Control of the fluoride to nitric acid ratios and/or positive additions of copper nitrate prevented this objectionable black coating.

In those instances wherein the fluoride to nitric acid ratio was sufficiently high to cause the formation of the objectionable black coating, addition of nitric acid while maintaining the fluoride concentration constant prevented the coating. In other words, lowering of the fluoride to nitric ratio to values within the limits set forth below effectively obviated the black coating problem. In addition, the black coating is also effectively eliminated by positive addition of copper as nitrate when operating at a fluoride to nitric acid ratio representing the maximum permissible while avoiding etching and/or haziness.

'Although it is not intended to limit the invention to any theory or mechanism, the foregoing is suggestive of an important role of nitrate ion in avoiding black coating formation. Thus, whether the nitrate ion concentration is increased by nitric acid addition or copper nitrate, upon maintenance of a proper minimum relative to fluoride the black coating is avoided. Analysis of the coating indicated the presence of aluminum, copper, silicon and smaller quantities of iron. Perhaps the nitrate ion is effective to maintain such constituents with the exception of copper in the oxidized (i. e. dissolved) state so that no deposition can occur, and copper deposition which then occurs alone has no adverse effect on the gains in specular reflectance which are obtainable.

In regard to copper deposition (or other metal electropositive to hydrogen e. g. silver), which occurs during the brightening operation, it may be visible or not discernible depending upon the concentration of the dissolved metal. In any event, it is readily removed by a subsequent nitric acid dip, even at room temperature. However, if the work is to be anodized in a conventional electrolyte, such as sulfuric acid, immediately after brightening the use of the nitric acid dip is obviated since the deposit is readily removed during anodizing without affecting the anodic coating formation.

The critical relative proportions of simple fluoride to nitric acid may be best defined by reference to the figure of the drawings which represents a plot of various concentrations of ammonium fluoride and nitric acid, expressed in grams per liter and ccs. of 70% (by weight) HNO; per liter, respectively. As indicated on Figure 1 aluminum ZS-I-llZ panels treated in solutions whose cornpositions fell above line AB exhibited decreases in specular reflectance with etching. Panels in solutions falling in the area to the right of line AB but outside of the closed areas exhibited measured gains in specular reflectance, but were visibly etched.

The aqueous acid brightening solutions of the present invention are those which contain fluoride ions and nitric acid equivalent to the relative concentrations of ammonium fluoride in grams per liter and nitric acid in cc. per liter (70% HNO: by weight) which are present within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG, and GC. Within these approximate limits the gains in specular reflectance are very acceptable while there is no etching or pitting. Some haziness is noticed in certain instances on panels treated by solutions lying in this range, particularly at the higher nitric and fluoride concentrations. Optimum results in terms of highest gain in specular reflectance and freedom from haziness as well as etching or pitting are obtained by solutions whose relative concentrations are within the area defined by solid lines CH, Hi, IF, PG, and GC.

Accordingly the diagram of the drawing accurately and definitely establishes the approximate limits of fluoride and nitric acid concentrations and their relative proportions for consistently reproducing the high gains in specular reflectance without etching, pitting or haziness in accordance with the invention. The diagram further contains contour lines (shown in dashed lines) which are drawn through those points in the areas, above-defined, corresponding to fluoride-nitric acid solutions which produce approximately equal gains in specular reflectance measured parallel to the rolling direction, the values of such gains being indicated for each contour line on the diagram.

The data for the diagram was obtained by treating test panels of 2S-Hl2 aluminum alloy having a copper content of 0.1% with 0.45% iron and 0.15% silicon, each panel being 4 x 6 x 0.084 inches. The panels were suspended in the various solutions, having the compositions indicated in Table I below, by means of an aluminum wire hook inserted through a hole punched in the center of one end of the panels. The wire hook in turn was suspended from a shaft in a freely turning bearing. Me-

chanical agitation of the solution caused the panel to revolve with the production of more evenly brightened work than in the case of stationary panels.

The time of treatment of the panels was five minutes, after which they were dipped in nitric acid, water rinsed and dried. The operating temperature of the bath was from 200 to 210 F. maintained by a hot plate or burner.

Specular reflectance values were obtained on the mill finish panels and again on the panels after treatment by means of a Henry A. Gardner glossmeter. The meter reading was adjusted to 100 with the instrument operating on a plate glass mirror. Then the specular reflectance (S. R.) of the panels before and after treatment was measured and read directly from the meter at a value under 100, the arbitrary S. R. for the standard. The gain in specular reflectance determined by difference is represented on the diagram by the contour lines.

In the tests from which the diagram was prepared, the

" copper content of the 2S-Hl2 aluminum alloy, namely 0.1%, produced a copper ion concentration not substantially less than about 0.001 gram/liter. For example, from the first panel treated in a gallon of fresh bath, 2 grams of metal dissolved, corresponding to 0.002 gram per gallon of dissolved copper (2X0.001) or about 0.0006 gram per liter. The first panel in a fresh bath was usually extremely well leveled and exhibited an optimum gain in specular reflectance, for example, a gain of 52 in specular reflectance measured parallel to the rolling direction was obtained with a solution containing 7.1 g./ l. NH4 and 30 cc./l. HNOa and no added copper salt.

The ranges of fluoride and nitric acid defined by the accompanying diagram were determined by means of the above-described specular reflectance measurements, and by visual observation to determine the presence or absence of etching and/ or haziness irrespective of measured gains in S. R. The coordinates for the lines on the diagram are set forth below in terms of solution composition.

TABLE I Solution composition Area CD, DE, EF, Area CH, HI, IF, AB F6, 0 F0, no

r-rNoi HNO; nNo3 A (10%). 31 (10%). 1 0%). cc./l. cell. cc./l.

1. 0 00.0 1a. 2 53.0 0.5 a. 0 14.3 00.0 0. 5 0. 0 11. 2 12.0 0. 5 13.0 10.0 04.0 1.3 21.0 22. 7 95. 0 2. 6 30. O 25.0 100. 0 0.0 53. 0 15.0 106.0 1.0 a 0 The favorable and unique results obtained by the solutions of the present invention due to presence of dissolved metal electropositive to hydrogen, for example, copper, as

a constituent of the brightening solutions at least in the very small amounts indicated is clearly demonstrated by the following data in which pure aluminum analyzing 99.8% aluminum and containing 0.01% copper was used as the test metal with and without positive additions of copper in the form of copper nitrate trihydrate. The test solution employed contained 7.1 g./l. NH4F and 30 cc./l. of 70% HNOa. As may be seen from the diagram, and as set forth above, this solution without copper addition produced optimum results on 2S-H12 alloy in terms of gains in specular reflectance 50), and smooth mirrorlike appearance with substantially no etching or haziness. The tests were performed as described above in connection with the data of Table I. Gains or losses in specular reflectance were measured both parallel and perpendicular to the rolling direction.

TABLE II Effect of copper salts on SR. gains for 99.8% aluminum (0.01% copper) [Brightening solution contained 7.1 g./1. NBA", 30 |nl./l. of

Thus, it may be seen that in the solution containing no added copper, a drastic loss in specular reflectance occurred compared to the mill finish surface, whereas a gain in specular reflectance occurred with as little as 0.01 g./l. of Cu(NOa)z3I-I2O, or about 0.0025 g./l. dissolved copper. Increased gains in S. R. were obtained with larger additions of copper.

The improvement obtained by positive additions of copper even when the aluminum metal contains as high as 0.1% copper is illustrated by the following data wherein copper nitrate trihydrate was added to solutions of the indicated composition in increasing amounts using 2S-H12 aluminum alloy (01% C11) as the work.

TABLE 111 Eflect of additions of copper salt on S.R. gains for ZS-HI2 (0.1% copper) [Brightening solution contained 7.1 g./l. NIIlF. 30 mL/l. of 70% nitric acid and copper salt as indicated.)

Gainsin S. R. (l iins in S. R.

(oncuutrntion of (711(NO112-3Hz0. glitter fi g g Direction 1) ircctlon 52 :ll 57 7 6t] 7t] 55 ("l 58 til The foregoing results indicate an improvement in S. R. gain for 2S-Hl2 by addition of copper in amounts up to about 0.1 gram/liter (about 0.4 gram/liter Cu(NOa)2- 31-120) in addition, the results also indicate excellent results are obtainable with the copper concentration in solution when such electropositive metal is derived solely from the aluminum being treated. Positive copper additions in large excess over about 0.1 gram per liter have no significantly adverse effect, nor do they produce beneficial effects, and in the interests of economy may be avoided.

For rapidity of action and most efficient brightening a temperature range of from about 180 F. to the boiling point is recommended, while 200 to 210 F. is regarded as optimum. The time of immersion of the metal in the solution is variable depending on concentrations and temperatures. In general, immersion times from about one to ten minutes are productive of desired results, while with solution concentrations and temperatures in the optimum range about five minutes is recommended.

The immersion time is also dependent to some extent on the previous condition of the metal being treated. For example with mill finish stock a longer immersion time is recommended, while with previously bufled material a shorter time of immersion produces better results.

In regard to brightening of buffed work, the solutions of the present invention are particularly advantageous in that the metal surface is favorably conditioned in such a manner that streaking and cloudiness frequently occurring on buffed work during subsequent anodizing is obviated. Also, the very dilute solutions within the preferred range of compositions given above are advantageously productive of excellent results on metal surfaces which have been previously buffed.

The process as generally employed is to prepare the aqueous solution with the indicated quantities of constituents, with or without copper salt addition, and heat the solution to the desired operating temperature, then immerse the metal for the desired time, after which the treated metal is given a nitric acid dip, unless it is to be immediately anodized. It is then water rinsed in either event and is suitably dried. Before immersion the work is degreased or cleaned in conventional inhibited alkaline cleaners or vapor degreasers. The bright dip bath or solution is also usually agitated to produce more uniform results. Air or mechanical agitation is suitable, the latter being preferred so as not to cool the heated solution.

Although the diagram of the drawings shows only ammonium fluoride and nitric acid, it is to be understood that the solution also contains ions of a metal electropositive to hydrogen in amount not substantially less than about 0.001 g./l. calculated as copper. Also, larger or smaller quantities of other substances may be present Without harmful effect. For example, glycerine in amounts up to as high as grams per liter exerts no adverse effect, although it is not productive of significant gains in specular reflectance over solutions without it. Also, other metal ions of metals electropositive to aluminum, but electronegative to hydrogen produced no adverse effect when present in amounts even higher than the copper content.

The process may be operated on a continuous or semi-continuous basis, or it may be conducted as a batch process. The components, that is, fluoride and nitric acid are gradually depleted and calculated additions of the compounds supplying the ions are periodically added to maintain proper concentrations. Where copper is positively added, it is also replenished by small additions of soluble salt.

The chemical brightening treatment may be conducted in any suitable tank or other apparatus provided with a lining impervious to the corrosive action of the nitric and hydrofluoric acids at the operating temperatures. Stainless steel of certain specifications is very satisfactory for this purpose. A particularly useful lining is Karbatea treated carbon product resistant to attack by all chemicals except those which are highly oxidizing. Karbate is manufactured in molded slabs, blocks and other shapes.

The heating of the solution may be accomplished by the use of Karbate tubes or heat exchangers. or, if electrical heating is desired Karbate shielded immersion heaters may be employed.

The chemical brightening compositions of the present invention advantageously provide an economical means for obtaining desired decorative finishes on aluminum and aluminum alloy surfaces which in appearance are bright, visibly smooth and clear, and have specular reflectances nearly comparable to those obtainable with the much more expensive electrobrightening. The solutions are well adapted for use on mill finish stock or previously buffed metal where highest specularity is the objective.

The highly dilute nature of the solutions insures very low drag out losses and low maintenance costs, while productive of consistently reproducible results when the solution compositions are maintained approximately within the concentrations and relative proportions of fluoride and nitric acid set forth above, and with the presence of the very small concentration of electropositive metal ion.

This application is a continuation-in-part of my copending applications Serial No. 101,692, filed June 27. 1949, now abandoned, and Serial No. 187,345, filed September 28, 1950, now Patent No. 2,620,265.

Various modifications of processing conditions and in solution compositions permitting the substantial realization of the results herein set forth are deemed within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys consisting essentially of fluoride ions and nitric acid in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, F6 and GC, and ions of a metal electropositive to hydrogen in amounts equivalent to not substantially less than about 0.001 gram per liter of copper.

2. A solution according to claim 1 in which the relative concentrations of ammonium fluoride and nitric acid lie within the area defined approximately in the accompanying diagram by the solid lines CH, HI, IF, PG and GC.

3. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys consisting essentially of hydrogen, nitrate and fluoride ions in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG and GC, and copper ions in amount not substantially less than about 0.001 gram per liter.

4. A solution according to claim 3 in which the relative concentrations of ammonium fluoride and nitric acid lie within the area defined approximately in the accompanying diagram by the solid lines CH, HI, IF, PG and GC.

5. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys having a copper content not substantially less than about 0.1% by weight, said solution consisting of fluoride ions and nitric acid in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG and GC.

6. A solution according to claim 5 in which the relative concentrations of ammonium fluoride and nitric acid lie within the area defined approximately in the accompanying diagram by the solid lines CH, HI, IF, PG and GC.

7. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys consisting essentially of nitric acid, and fluoride ions in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG and GC, and as a third positively added constituent, ions of a metal electropositive to hydrogen in concentration equivalent to the copper 10 ion concentration produced by addition of not substantially less than about 0.03 gram per liter CLl(NO3)2-3H20.

8. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys consisting essentially of nitric acid, and fluoride ions in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG and GC, and as a third positively added constituent, copper ions in amount equivalent to that produced by addition of not substantially less than about 0.03 gram Cu(NOs)z-3Hz0.

9. A solution according to claim 8 wherein there is added at least about 0.4 gram per liter Cu(NOa)z-3H2O.

10. An aqueous acid solution for the chemical brightening of aluminum and aluminum alloys consisting essentially of nitric acid and ammonium fluoride in relative concentrations lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG, and GC, said solution also containing copper ions in concentration not substantially less than about 0.001 gram per liter.

11. A solution according to claim 10 wherein the relative concentrations of nitric acid and ammonium fluoride lie within the area defined approximately in the accompanying diagram by the solid lines CH, HI, IF, PG and GC.

12. A solution according to claim 11 wherein the solution contains copper ions in amount equivalent to that produced by addition of not substantially less than 0.4 gram per liter Cu(NOs)2- 3H2O.

13. A process for chemically brightening aluminum and aluminum alloys which comprises contacting the metal surfaces with a heated aqueous acid solution consisting essentially of fluoride ions and nitric acid in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CD, DE, EF, PG, and GC, and ions of a metal electropositive to hydrogen in concentration equivalent to not substantially less than 0.001 gram per liter of copper, for a time sufficient to brighten the metal.

14. A process according to claim 13 in which the solution temperature is maintained from about F. to boiling.

15. A process for chemically brightening aluminum and aluminum alloys which comprises immersing the metal in an aqueous acid solution maintained at from 180 F. to boiling and consisting essentially of fluoride ions and nitric acid in concentrations equivalent to the relative concentrations of ammonium fluoride and nitric acid lying within the area defined approximately in the accompanying diagram by the solid lines CH, HI, IF, PG and GC, said solution containing also copper ions in amount not substantially less than 0.001 gram per liter for a time suflicient to brighten the metal, and thereafter rinsing and drying the metal.

16. A process according to claim 15 in which the metal is treated with nitric acid to remove the deposited copper from the brightened surfaces.

17. A process according to claim 15 in which the copper ion concentration is not substantially less than about 0.1 gram per liter by addition of not less than about 0.4 gram per liter of Cu(NOa)2- 3H2O.

18. A process according to claim 15 in which the temperature of the solution is maintained from about 200 to about 210 F.

19. A process according to claim 15 wherein the solution contains copper ions at least in amount equivalent to that produced by addition of not substantially less than about 0.03 gram per liter Cu(NOs)2-3H2O.

References Cited in the file of this patent UNITED STATES PATENTS 2,593,449 Hesch Apr. 22, 1952 

15. A PROCESS FOR CHEMICALLY BRIGHTENING ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES IMMERSING THE METAL IN AN AQUEOUS ACID SOLUTION MAINTAINED AT FROM 180* F. TO BOILING AN CONSISTING ESSENTIALLY OF FLUORIDE IONS AND NITRIC ACID IN CONCENTRATIONS EQUIVALENT TO THE RELATIVE CONCENTRATIONS OF AMMONIUM FLUORIDE AND NITRIC ACID LYING WITHIN THE AREA DEFINED APPROXIMATELY IN THE ACCOMPANYING DIAGRAM BY THE SOLID LINES CH,HI,IF,GF AND GC, SAID SOLUTION CONTAINING ALSO COPPER IONS IN AMOUNT NOT SUBSTANTIALLY LESS THA 0.001 GRAM PER LITER FOR A TIME SUFFICIENT TO BRIGHTEN THE METAL, AND THEREAFTER RINSING AND DRYING THE METAL. 